I have very recent experience with this. I've had a Pass XP10 pre amp running my Pass 250.8 through a JL Audio CR1 active crossover. I have a purpose built very solid, very tight and quiet listening room. I could hear a very faint hum at 7 am when the house was totally quiet, otherwise I never heard it. I just upgraded to an Audio Research LS28 tube preamp - surprisingly it's dead quiet !
How much hum is normal from a new tube amp
I just bought a great sounding Lab12 Suara KT120 tube amp. The issue I am having is a hum that is coming from the transformers. I can hear it with no music playing faintly from my listening position 12 ft. away. The question is, should I return it, or is this normal acceptable.